1. Field of the Invention
The present invention relates to an optical amplification module for amplifying signal light, an optical amplification apparatus, and an optical communications system including the optical amplification apparatus.
2. Related Background Art
Wavelength division multiplexing (WDM) optical communications systems are systems transmitting signal light (multiplexed signal light) in which a plurality of channels included in a predetermined signal wavelength band are multiplexed, thus making it possible to transmit/receive a large volume of information. For further increasing the amount of information which can be transmitted/received, using not only multiplexed signal light included in C band (1530 nm to 1565 nm) but also that included in L band (1565 nm to 1625 nm) has been under study.
Accordingly, optical amplification apparatus employed in optical communications systems are required to realize signal light amplification not only in C band but also in L band. Known as such an optical amplification apparatus is one (EDFA: Erbium Doped Fiber Amplifier) employing an optical fiber (EDF: Erbium Doped Fiber) made of silica-based host glass whose optical waveguide region is doped with Er element. In this optical amplification apparatus, pumping light having a wavelength (1.48 μm or 0.98 μm) capable of pumping Er element is fed to the EDF, which then amplifies signal light in C band or L band.
For example, an optical amplification apparatus disclosed in A. Mori, et al., ECOC 1997, Tech. Dig., p. 135 (literature 1) employs, as an optical amplification medium, a silica-based EDF codoped with Al, thereby yielding a flat gain up to a long wavelength of about 1600 nm. However, the flat wavelength band is too narrow for this optical amplification apparatus to collectively amplify the multiplexed signal light in L band. The above-mentioned literature 1 also proposes an optical amplification apparatus employing, as an optical amplification medium, an EDF made of tellurite-based host glass so as to attain a flat gain toward a longer wavelength side. However, this optical amplification apparatus may not be practical since tellurite glass incurs a fear of thermal damages.
An optical amplification apparatus disclosed in A. J. G. Ellison, et al., OFC 2001, TuA2 (literature 2) employs, as an optical amplification medium, a multicomponent silica-based EDF containing Sb element. However, this optical amplification apparatus may not be practical since Sb element is toxic.
Further, optical amplification apparatus which are practical as optical amplification apparatus media in that they incur no problems of thermal damages and toxicity have been proposed. For example, though the composition of a silica-based EDF disclosed in I. P. Byriel, et al., ECOC2001, Tu. L. 3.5 (literature 3) is unclear, silica-based EDFs disclosed in Kakui, et al., The 2002 IEICE General Conference C-3-28 (literature 4) and S. Tanaka, et al., OFC2002, Tech. Dig., ThJ3 (literature 5) are codoped with P element and Al element. The EDFs disclosed in literatures 3 to 5 attain a gain up to a long wavelength of about 1620 nm. However, their gain is remarkably low near a wavelength of 1580 nm, thus deteriorating a gain flatness which is important in WDM transmissions.
Here, the gain flatness of an EDF can be evaluated by a relative gain non-uniformity which will be explained with reference to FIG. 1. FIG. 1 is a typical gain spectrum for explaining the relative gain non-uniformity of an EDF. As shown in FIG. 1, the gain spectrum of an EDF roughly has two maximum gain values and one minimum gain value within a wavelength band exhibiting a gain. Let Gmin (dB) be the minimum gain value, and AG be the difference between the maximum gain value Gmax (dB) and the minimum gain value Gmin (dB). Let the ratio (ΔG/Gmin) of the difference ΔG to the minimum gain value Gmin (dB) represent the relative gain non-uniformity. The wavelength band yielding a gain not lower than the minimum gain value Gmin (dB) will be referred to as an effective signal wavelength region.
In terms of the gain flatness evaluation according to the relative gain non-uniformity defined in the foregoing, the EDFs disclosed in literatures 3 to 5 yield relative gain non-uniformities of about 25%, greater than 30%, and about 25%, respectively. If the relative gain non-uniformity of an EDF is too large, the insertion loss of optical filters inserted for gain equalization must increase, thereby deteriorating the pumping efficiency and noise figure.
For example, the gain of optical amplification apparatus required in main lines on the ground is about 30 dB. EDFs employed in such optical amplification apparatus are assumed to have a relative gain non-uniformity of 25%. In ground main line systems, an optical amplification apparatus incorporates therein a dispersion-compensating optical fiber in addition to an EDF in general, whereas the dispersion-compensating optical fiber typically exhibits a loss of about 10 dB. Also, other passive optical components (e.g., optical couplers and optical isolators) are inserted into the optical amplification apparatus, and yield an insertion loss of about 6 dB in total. Here, the gain to be realized by the EDF may become as high as 46 dB (=30 dB+10 dB+6 dB). Since the EDF has a relative gain non-uniformity of 25%, the peak insertion loss of optical filters inserted for gain equalization may be as high as 11.5 dB (=46 dB×0.25) at that time, which is on a par with the loss of the dispersion-compensating optical fiber. This may exert a large adverse effect on the pumping efficiency and noise figure.
Meanwhile, silica-based EDFs of C-band optical amplification apparatus which have already become widespread exhibit a relative gain non-uniformity on the order of 13% to 19% though depending on the kind and concentration of elements with which the EDFs are doped. FIG. 2 is a gain spectrum of an Al-codoped silica-based EDF. In FIG. 2, curves G2010, G2020, G2030, and G2040 show respective gain spectra with Al codopant concentrations of 1 wt %, 2.5 wt %, 3.5 wt %, and 5 wt %. As can be seen from FIG. 2, the relative gain non-uniformity of the EDF decreases as the Al codopant concentration is higher, and is on the order of 13% to 19%. Therefore, a value on this order becomes a target value for L-band optical amplification apparatus as well.